“Among” is appropriate to describe broad relationships when more than two things are involved:

Deforestation is among the world’s environmental problems.

Laboratory experiments identified general relationships among crushing parameters, product size, and coal properties.

“Between” is used to describe specific relationships involving only two things:

A satisfactory agreement was reached between the two countries.

Current usage also permits “between” when each entity is considered individually or severally in relation to the others:

Between them, each client agreed that this solution was best.

Ratios were calculated between each of the four fixed-location sites and two moveable sites.